Furnaces and refractories - RETScreen International

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©

UNEP 2006

1

Training Session on Energy
Equipment


Furnaces and
Refractories

Presentation from the

“Energy Efficiency Guide for Industry in Asia”

www.energyefficiencyasia.org



©

UNEP 2006

2

Training Agenda: Steam

Introduction

Type of furnaces and refractory
materials

Assessment of furnaces

Energy efficiency opportunities


©

UNEP 2006

3

Introduction


Equipment to melt metals


Casting


Change shape


Change properties


Type of fuel important


Mostly liquid/gaseous fuel or electricity


Low efficiencies due to


High operating temperature


Emission of hot exhaust gases

What is a Furnace?


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UNEP 2006

4

Introduction

Furnace Components

(The Carbon Trust)

Furnace chamber:
constructed of
insulating materials

Hearth: support or
carry the steel.
Consists of
refractory materials

Burners: raise or
maintain chamber
temperature

Chimney:
remove
combustion
gases

Charging & discharging doors for
loading & unloading stock

Charging & discharging doors for
loading & unloading stock


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5

Introduction

Materials that


Withstand high temperatures and sudden
changes


Withstand action of molten slag, glass, hot
gases etc


Withstand load at service conditions


Withstand abrasive forces


Conserve heat


Have low coefficient of thermal expansion


Will not contaminate the load


What are Refractories:


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UNEP 2006

6

Introduction

Refractories

Refractory lining of a
furnace arc




Refractory walls of a
furnace interior with
burner blocks

(BEE India, 2005)


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UNEP 2006

7

Introduction


Melting point


Temperature at which a ‘test pyramid’ (cone)
fails to support its own weight


Size


Affects stability of furnace structure


Bulk density


Amount of refractory material within a
volume (kg/m3)


High bulk density = high volume stability,
heat capacity and resistance

Properties of Refractories


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8

Introduction


Porosity


Volume of open pores as % of total refractory
volume


Low porosity = less penetration of molten
material


Cold crushing strength


Resistance of refractory to crushing


Creep at high temperature


Deformation of refractory material under
stress at given time and temperature

Properties of Refractories


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UNEP 2006

9

Introduction


Pyrometric cones


Used in ceramic industries

to test ‘refractoriness’ of

refractory bricks


Each cone is mix of oxides

that melt at specific

temperatures

Properties of Refractories


Pyrometric Cone Equivalent (PCE)


Temperature at which the refractory brick
and

the cone bend


Refractory cannot be used above this temp

(BEE India, 2004)


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10

Introduction


Volume stability, expansion &
shrinkage


Permanent changes during refractory service
life


Occurs at high temperatures


Reversible thermal expansion


Phase transformations during heating and
cooling

Properties of Refractories


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11

Introduction


Thermal conductivity


Depends on composition and silica content


Increases with rising temperature


High thermal conductivity:


Heat transfer through brickwork required


E.g. recuperators, regenerators


Low thermal conductivity:


Heat conservation required (insulating
refractories)


E.g. heat treatment furnaces

Properties of Refractories


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UNEP 2006

12

Training Agenda: Steam

Introduction

Type of furnaces and refractory
materials

Assessment of furnaces

Energy efficiency opportunities


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UNEP 2006

13

Type of Furnaces and Refractories


Type of Furnaces


Forging furnaces


Re
-
rolling mill furnaces


Continuous reheating furnaces


Type of Refractories


Type of Insulating Materials


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14

Type of Furnaces and Refractories

Classification Combustion Furnaces

Classification

method

Types

and

examples

1. Type of fuel used

Oil
-
fired

Gas
-
fired

Coal
-
fired

2. Mode of charging materials

Intermittent

/

Batch

Periodical


Forging


Re
-
rolling

(batch/pusher)


Pot

Continuous


Pusher


Walking

beam


Walking

hearth


Continuous

recirculating

bogie

furnaces


Rotary

hearth

furnaces

3. Mode of heat transfer

Radiation (open fire place)

Convection

(heated

through

medium)

4. Mode of waste heat
recovery

Recuperative

Regenerative


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Type of Furnaces and Refractories


Used to preheat billets/ingots


Use open fireplace system with
radiation heat transmission


Temp 1200
-
1250 oC


Operating cycle


Heat
-
up time


Soaking time


Forging time


Fuel use: depends on material and
number of reheats

Forging Furnace


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16

Type of Furnaces and Refractories


Box type furnace


Used for heating up scrap/ingots/billets


Manual charge / discharge of batches


Temp 1200 oC


Operating cycle: heat
-
up, re
-
rolling


Output 10
-

15 tons/day


Fuel use: 180
-
280 kg coal/ton material

Re
-
rolling Mill Furnace


Batch type


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UNEP 2006

17

Type of Furnaces and Refractories


Not batch, but continuous charge and
discharge


Temp 1250 oC


Operating cycle: heat
-
up, re
-
rolling


Output 20
-
25 tons/day


Heat absorption by material is slow,
steady, uniform

Re
-
rolling Mill Furnace


Continuous pusher type


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18

Type of Furnaces and Refractories


Continuous material flow


Material temp 900


1250 oC


Door size minimal to avoid air
infiltration


Stock kept together and pushed


Pusher type furnaces


Stock on moving hearth or structure


Walking beam, walking hearth, continuous
recirculating bogie, rotary hearth furnaces

Continuous Reheating Furnaces


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19

Type of Furnaces and Refractories

1. Pusher Furnace


Pushers on ‘skids’ (rails) with water
-
cooled
support push the stock


Hearth sloping towards discharge end


Burners at discharge

end or top and/or

bottom


Chimney with

recuperator for

waste heat recovery

(The Carbon Trust, 1993)

Continuous Reheating Furnaces


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20

Type of Furnaces and Refractories

2. Walking Beam Furnace


Stock placed on stationary ridges


Walking beams raise the stock and move forwards


Walking beams lower stock onto stationary ridges
at exit


Stock is removed


Walking beams

return to furnace

entrance


(The Carbon Trust, 1993)

Continuous Reheating Furnaces


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21

Type of Furnaces and Refractories

3. Walking Hearth Furnace


Refractory blocks extend through hearth
openings


Stock rests on fixed refractory blocks


Stock transported

in small steps

‘walking the hearth’


Stock removed

at discharge end



(The Carbon Trust, 1993)

Continuous Reheating Furnaces


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22

Type of Furnaces and Refractories

4. Continuous Recirculating Bogie
Furnace


Shape of long and narrow tunnel


Stock placed on bogie (cart with wheels) with
refractory hearth


Several bogies

move like train


Stock removed

at discharge end


Bogie returned

to entrance



(The Carbon Trust, 1993)

Continuous Reheating Furnaces


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23

Type of Furnaces and Refractories

5. Rotary Hearth Furnace


Walls and roof remain stationary


Hearth moves in circle on rollers


Stock placed on hearth


Heat moves in

opposite direction

of hearth


Temp 1300
o
C



(The Carbon Trust, 1993)

Continuous Reheating Furnaces


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24

Type of Furnaces and Refractories

Classification of Refractories

Classification method

Examples

Chemical composition

ACID, which readily combines with bases

Silica, Semisilica, Aluminosilicate

BASIC, which consists mainly of metallic
oxides that resist the action of bases

Magnesite, Chrome
-
magnesite, Magnesite
-
chromite, Dolomite

NEUTRAL, which does not combine with
acids nor bases

Fireclay bricks, Chrome, Pure Alumina

Special

Carbon, Silicon Carbide, Zirconia

End use

Blast furnace casting pit

Method of manufacture

Dry press process, fused cast, hand
moulded, formed normal, fired or chemically
bonded, unformed (monolithics, plastics,
ramming mass, gunning castable, spraying)


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25

Type of Furnaces and Refractories


Common in industry: materials available and
inexpensive


Consist of aluminium silicates


Decreasing melting point (PCE) with increasing
impurity and decreasing AL2O3

Fireclay Refractories


45
-

100% alumina


High alumina % = high refractoriness


Applications: hearth and shaft of blast furnaces,
ceramic kilns, cement kilns, glass tanks

High Alumina Refractories


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26

Type of Furnaces and Refractories


>93% SiO2 made from quality rocks


Iron & steel, glass industry


Advantages: no softening until fusion point is
reached; high refractoriness; high resistance to
spalling, flux and slag, volume stability

Silica Brick


Chemically basic: >85% magnesium oxide


Properties depend on silicate bond concentration


High slag resistance, especially lime and iron

Magnesite


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Type of Furnaces and Refractories


Chrome
-
magnesite


15
-
35% Cr2O3 and 42
-
50% MgO


Used for critical parts of high temp furnaces


Withstand corrosive slags


High refractories


Magnesite
-
chromite


>60% MgO and 8
-
18% Cr2O3


High temp resistance


Basic slags in steel melting


Better spalling resistance

Chromite Refractories


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Type of Furnaces and Refractories


Zirconium dioxide ZrO2


Stabilized with calcium, magnesium, etc.


High strength, low thermal conductivity, not
reactive, low thermal loss


Used in glass furnaces, insulating refractory

Zirconia Refractories


Aluminium oxide + alumina impurities


Chemically stable, strong, insoluble, high
resistance in oxidizing and reducing atmosphere


Used in heat processing industry, crucible shaping

Oxide Refractories (Alumina)


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29

Type of Furnaces and Refractories


Single piece casts in equipment shape


Replacing conventional refractories


Advantages


Elimination of joints


Faster application


Heat savings


Better spalling resistance


Volume stability


Easy to transport, handle, install


Reduced downtime for repairs

Monolithics


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30

Type of Furnaces and Refractories


Material with low heat conductivity:
keeps furnace surface temperature
low


Classification into five groups


Insulating bricks


Insulating castables and concrete


Ceramic fiber


Calcium silicate


Ceramic coatings (high emissivity coatings)

Insulating Materials Classification


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Type of Furnaces and Refractories


Consist of


Insulation materials used for making piece
refractories


Concretes contain Portland or high
-
alumina
cement


Application


Monolithic linings of furnace sections


Bases of tunnel kiln cars in ceramics
industry

Castables and Concretes


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32

Type of Furnaces and Refractories


Thermal mass insulation materials


Manufactured by blending alumina
and silica


Bulk wool to make insulation
products


Blankets, strips, paper, ropes, wet felt etc


Produced in two temperature grades

Ceramic Fibers


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33

Type of Furnaces and Refractories


Low thermal conductivity


Light weight


Lower heat storage


Thermal shock resistant


Chemical resistance


Mechanical resilience


Low installation costs


Ease of maintenance


Ease of handling


Thermal efficiency

Ceramic Fibers

Remarkable properties and benefits


Lightweight furnace


Simple steel fabrication
work


Low down time


Increased productivity


Additional capacity


Low maintenance costs


Longer service life


High thermal efficiency


Faster response


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34

Type of Furnaces and Refractories


Emissivity: ability to absorb and
radiate heat


Coatings applied to interior furnace
surface:


emissivity stays constant


Increase emissivity from 0.3 to 0.8


Uniform heating and extended refractory life


Fuel reduction by up to 25
-
45%

High Emissivity Coatings


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UNEP 2006

35

Type of Furnaces and Refractories

High Emissivity Coatings

(BEE India, 2005)


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UNEP 2006

36

Training Agenda: Steam

Introduction

Type of furnaces and refractory
materials

Assessment of furnaces

Energy efficiency opportunities


©

UNEP 2006

37

Assessment of Furnaces

Heat Losses Affecting Furnace
Performance


FURNACE
Flue gas
Moisture in fuel
Openings in furnace
Furnace surface/skin
Other losses
Heat input
Heat in stock
Hydrogen in fuel
FURNACE
Flue gas
Moisture in fuel
Openings in furnace
Furnace surface/skin
Other losses
Heat input
Heat in stock
Hydrogen in fuel

©

UNEP 2006

38

Assessment of Furnaces

Instruments to Assess Furnace
Performance

Parameters

to be measured

Location of

measurement

Instrument

required

Required

Value

Furnace soaking zone
temperature (reheating
furnaces)

Soaking zone and side
wall

Pt/Pt
-
Rh thermocouple
with indicator and
recorder

1200
-
1300
o
C

Flue gas temperature

In duct near the discharge
end, and entry to
recuperator

Chromel Alummel
Thermocouple with
indicator

700
o
C

max
.


Flue gas temperature

After recuperator

Hg in steel thermometer

300
o
C

(max)


Furnace hearth pressure
in the heating zone

Near charging end and
side wall over the hearth

Low pressure ring gauge

+
0
.
1

mm

of

Wc


Oxygen in flue gas

In duct near the discharge
end

Fuel efficiency monitor for
oxygen and temperature

5% O
2

Billet temperature

Portable

Infrared pyrometer or
optical pyrometer

-


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UNEP 2006

39

Assessment of Furnaces

Direct Method


Thermal efficiency of furnace

= Heat in the stock / Heat in fuel
consumed for heating the stock


Heat in the stock Q:



Q = m x Cp (t1


t2)

Calculating Furnace Performance

Q = Quantity of heat of stock in kCal

m = Weight of the stock in kg

Cp= Mean specific heat of stock in kCal/kg oC

t1 = Final temperature of stock in oC

t2 = Initial temperature of the stock before it enters the furnace in oC


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40

Assessment of Furnaces

Direct Method
-

example


Heat in the stock Q
=


m x Cp (t1


t2)


6000 kg X 0.12 X (1340


40)


936000 kCal


Efficiency =


(heat input / heat output) x 100


[936000 / (368 x 10000) x 100 = 25.43%


Heat loss = 100%
-

25% = 75%

Calculating Furnace Performance

m = Weight of
the stock = 6000
kg

Cp= Mean
specific heat of
stock = 0.12
kCal/kg oC

t1 = Final
temperature of
stock = 1340 oC

t2 = Initial
temperature of
the stock = 40 oC

Calorific value of
oil = 10000
kCal/kg

Fuel consumption
= 368 kg/hr


©

UNEP 2006

41

Assessment of Furnaces

Indirect Method

Heat losses

a)
Flue gas loss



= 57.29 %

b)
Loss due to moisture in fuel


= 1.36 %

c)
Loss due to H2 in fuel


= 9.13 %

d)
Loss due to openings in furnace

= 5.56 %

e)
Loss through furnace skin


= 2.64 %

Total losses




= 75.98 %

Furnace efficiency =


Heat supply minus total heat loss


100%


76% = 24%

Calculating Furnace Performance


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Assessment of Furnaces

Typical efficiencies for industrial furnaces

Calculating Furnace Performance

Furnace type

Thermal efficiencies (%)

1
)

Low

Temperature

furnaces




a
.

540



980

o
C

(Batch

type)


20
-
30

b
.

540



980

o
C

(Continous

type)


15
-
25

c
.

Coil

Anneal

(Bell)

radiant

type


5
-
7

d
.

Strip

Anneal

Muffle


7
-
12

2
)

High

temperature

furnaces




a
.

Pusher,

Rotary


7
-
15

b
.

Batch

forge


5
-
10

3
)

Continuous

Kiln




a
.

Hoffman


25
-
90

b
.

Tunnel


20
-
80

4
)

Ovens




a
.

Indirect

fired

ovens

(
20

o
C


370

o
C
)


35
-
40

b
.

Direct

fired

ovens

(
20

o
C


370

o
C
)


35
-
40


©

UNEP 2006

43

Training Agenda: Steam

Introduction

Type of furnaces and refractory
materials

Assessment of furnaces

Energy efficiency opportunities


©

UNEP 2006

44

Energy Efficiency Opportunities

1.
Complete combustion with minimum excess air

2.
Proper heat distribution

3.
Operation at the optimum furnace temperature

4.
Reducing heat losses from furnace openings

5.
Maintaining correct amount of furnace draft

6.
Optimum capacity utilization

7.
Waste heat recovery from the flue gases

8.
Minimize furnace skin losses

9.
Use of ceramic coatings

10.
Selecting the right refractories


©

UNEP 2006

45

Energy Efficiency Opportunities


Importance of excess air


Too much: reduced flame temp, furnace
temp, heating rate


Too little: unburnt in flue gases, scale losses


Indication of excess air: actual air /
theoretical combustion air


Optimizing excess air


Control air infiltration


Maintain pressure of combustion air


Ensure high fuel quality


Monitor excess air

1. Complete Combustion with
Minimum Excess Air


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UNEP 2006

46

Energy Efficiency Opportunities

When using burners


Flame should not touch or be obstructed


No intersecting flames from different burners


Burner in small furnace should face upwards
but not hit roof


More burners with less capacity (not one big
burner) in large furnaces


Burner with long flame to improve uniform
heating in small furnace

2. Proper Heat Distribution


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UNEP 2006

47

Energy Efficiency Opportunities


Operating at too high temperature:
heat
loss, oxidation, decarbonization, refractory stress


Automatic controls eliminate human
error

3. Operate at Optimum Furnace
Temperature

Slab

Reheating

furnaces


1200
o
C


Rolling

Mill

furnaces


1200
o
C


Bar

furnace

for

Sheet

Mill


800
o
C


Bogie type annealing furnaces

650
o
C


750
o
C



©

UNEP 2006

48

Energy Efficiency Opportunities


Heat loss through openings


Direct radiation through openings


Combustion gases leaking through the openings


Biggest loss: air infiltration into the furnace


Energy saving measures


Keep opening small


Seal openings


Open furnace doors less frequent and shorter

4. Reduce Heat Loss from Furnace
Openings


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49

Energy Efficiency Opportunities


Negative pressure in furnace: air
infiltration


Maintain slight positive pressure


Not too high pressure difference: air
ex
-
filtration


Heat loss only about 1% if furnace
pressure is controlled properly!

5. Correct Amount of Furnace Draft


©

UNEP 2006

50

Energy Efficiency Opportunities


Optimum load


Underloading: lower efficiency


Overloading: load not heated to right temp


Optimum load arrangement


Load receives maximum radiation


Hot gases are efficiently circulated


Stock not placed in burner path, blocking flue
system, close to openings


Optimum residence time


Coordination between personnel


Planning at design and installation stage

6. Optimum Capacity Utilization


©

UNEP 2006

51

Energy Efficiency Opportunities


Charge/Load pre
-
heating


Reduced fuel needed to heat them in furnace


Pre
-
heating of combustion air


Applied to compact industrial furnaces


Equipment used: recuperator, self
-
recuperative burner


Up to 30% energy savings


Heat source for other processes


Install waste heat boiler to produce steam


Heating in other equipment (with care!)

7. Waste Heat Recovery from Flue Gases


©

UNEP 2006

52

Energy Efficiency Opportunities


Choosing appropriate refractories


Increasing wall thickness


Installing insulation bricks (= lower
conductivity)


Planning furnace operating times


24 hrs in 3 days: 100% heat in refractories
lost


8 hrs/day for 3 days: 55% heat lost

8. Minimum Furnace Skin Loss


©

UNEP 2006

53

Energy Efficiency Opportunities


High emissivity coatings


Long life at temp up to 1350 oC


Most important benefits


Rapid efficient heat transfer


Uniform heating and extended refractory life


Emissivity stays constant


Energy savings: 8


20%

9. Use of Ceramic Coatings


©

UNEP 2006

54

Energy Efficiency Opportunities

Selection criteria


Type of furnace


Type of metal charge


Presence of slag


Area of application


Working temperatures


Extent of abrasion
and impact

10. Selecting the Right Refractory



Structural load of
furnace


Stress due to temp
gradient & fluctuations


Chemical compatibility


Heat transfer & fuel
conservation


Costs


©

UNEP 2006

55

Training Session on Energy
Equipment

Furnaces and
Refractories



THANK YOU

FOR YOUR ATTENTION





©

UNEP 2006

56

Disclaimers and References


This PowerPoint training session was prepared as part of
the project “Greenhouse Gas Emission Reduction from
Industry in Asia and the Pacific” (GERIAP). While
reasonable efforts have been made to ensure that the
contents of this publication are factually correct and
properly referenced, UNEP does not accept responsibility for
the accuracy or completeness of the contents, and shall not
be liable for any loss or damage that may be occasioned
directly or indirectly through the use of, or reliance on, the
contents of this publication. © UNEP, 2006.


The GERIAP project was funded by the Swedish
International Development Cooperation Agency (Sida)


Full references are included in the textbook chapter that is
available on www.energyefficiencyasia.org